The present invention relates to compositions and methods for the mitigation of aluminum and ferric precipitates in subterranean formations during acidizing operations.
Acidizing operations may be used to cleanup a damaged wellbore or subterranean formation (e.g., removal of scale) and/or in fracturing a formation that is susceptible to dissolution by the acid. One acidizing operation, known as “matrix acidizing,” comprises injecting an acidizing fluid into the formation at a pressure below the fracture gradient pressure (i.e., below the pressure sufficient to create or extend a fracture within the subterranean formation). Accordingly, in matrix acidizing, the acid source within the acidizing fluid is permitted to contact and react with the matrix of the subterranean formation so as to enhance its permeability, but the formation is not fractured. In some instances, after an acidizing treatment is completed, the water and salts dissolved therein may be recovered by producing them to the surface, e.g., “flowing back” the well, thereby leaving a desirable amount of voids within the formation, which enhances the formation's permeability and increases the rate at which hydrocarbons may subsequently be produced from the formation.
In each instance, minerals are dissolved by the acid into salts and removed from the subterranean formation, thereby enhancing permeability therethrough. However, when the concentration of salt ions in the acid is above saturation, precipitates can form that plug the voids in the subterranean formation. Therefore, acidizing operations often circulate large volumes of fluid and minimize treatment times to mitigate precipitate formation. In such acidizing operations, several treatments may be required to achieve a desired cleanup or fracturing result. Accordingly, acidizing operations are often energy intensive and costly, especially if multiple treatments are needed.
To mitigate precipitate formation and allow for longer treatment times, traditional chelating agents like ethylenediaminetetraacetic acid (“EDTA”) and nitrilotriacetic acid (“NTA”) has been added to acidizing fluids. These chelating agents sequester ions from the minerals and salts dissolved in the acid, thereby allowing for higher concentrations of ions to be dissolved in the acid. However, traditional chelating agents decrease efficacy as the pH increases, and in some instances, are ineffective above about pH 2.5-3. Therefore, lower pHs and stronger acids are often required for acidizing operations.
Further, in some subterranean formations, acidizing operations can produce high concentrations of trivalent ions like Fe3+ and Al3+, which have lower saturation points than Ca2+, Mg2+, K+, and Na+ ions. Traditional chelating agents often have higher binding energies to monovalent and divalent ions like Ca2+, Mg2+, K+, and Na+. Therefore, in the presence of a diverse ion mixture often found in the acidic dissolution of subterranean formations, monovalent and divalent ions will preferentially be sequestered by traditional chelating agents rendering them less effective to ineffective for trivalent ions. Because the saturation point for trivalent ions is often lower than monovalent and trivalent ions, trivalent ions precipitate more readily and continue to be a hindrance in developing more efficient and effective acidizing treatments.